1. Field of the Invention
This invention relates to an isocyanate-curing coating composition suitable for uses in automobile repair, coating of plastics, wood, building materials, and the like and a method for the application thereof. More particularly, this invention relates to a low-temperature grade isocyanate-curing coating material which excels in mechanical properties, physical properties such as weatherability and coating and finishing properties, and chemical properties and a method for the application of the coating material. The coating material of this invention, emits little organic solvent while being applied to a surface.
2. Prior Art Statement
The polyisocyanate-curing two-component urethane coating material is widely used as a low-temperature curing coating material with excellent coating properties. From the point of ensurounmental protection, however, it is important to decrease in the amount of organic solvent emitted from such coating materials during application a surface. This has led to the establishment of regulations such as Rule 66 in the United States of America, TA-LUFT in Germany, and prefectural ordinances in Osaka, Japan.
Since the two-component urethane coating materials comes under these regulations, so-called high-solid urethane coating materials are being developed which are high in solid components and emit little organic solvent during application. As a means to produce such coating materials conforming to these regulations, methods of reducing the molecular weights of hydroxyl group-containing polymers, isocyanate compounds, etc. have been tested.
Since these methods are merely aimed at reducing the molecular weights of existing polymers, the films formed of the coating materials produced by these methods are markedly poorer in such coating properties as mechanical property and weatherability than the films formed of the conventional coating materials. Among the methods for producing coating materials that achieve the object mentioned above is a method which comprises adding an organic solvent to a hydroxyl group-containing polymer and an isocyanate compound. Concerning this method, the feasibility of attaining a high-solid composition by adding a compound containing two or three hydroxyl groups in the molecular unit and having a molecular weight of several hundreds as a reactive diluent instead of adding the organic solvent has been also studied. The films formed of the compositions produced by these methods, however, are extremely inferior in such coating properties as mechanical property and weatherability to the films formed of the conventional coating materials and, therefore, are not practicable.
A known method of decreasing the amount of an organic solvent emitted from a coating material during its application to a surface is to lower the degree of dilution of the coating material with the solvent in preparation for the application to a surface. By this method, the coating material is diluted with an organic solvent to the optimum viscosity prior to the application and is then put to use. Specifically, this method consists in decreasing the amount of the diluting solvent to such an extent as to lower the viscosity in coating from the standard level of 5-25 seconds (the time required for a drop of the diluted coating material to fall from Ford Cup No. 4) to above 30 seconds. This is the easiest way of accomplishing a high-solid composition and cannot impair the coating properties because it does not alter the composition of solid components. Since the coating material of this method has high viscosity, however, it does not produce the perfect appearance sought in such applications as automobile repair and coating of plastics, wood and building materials for which the two-component urethane coating materials are widely used.
A high-solid two-component urethane coating material which is fully practicable remains yet to be developed.
The present inventors conducted a study with a view to developing a high-solid isocyanate-curing coating material which does not experience degradation of coating properties, such as mechanical properties, weatherability and the coat finishing property. As a result, they learned that an isocyanate-curing coating material experiences a peculiar phenomenon of abruptly falling in viscosity when its temperature thereof rises beyond 35.degree. C. This invention was accomplished as a result.